Electroacoustic translator, including impedance matching



.Sept 6, 1949.

P. BEST f TRANSLATOR, INCLUDING F. ELECTROACOUSTIC IMPEDANCE MATCHING Filed NQV. 27, 1945 Patented Sept. 6, 1949 UNITEDA STATES PATENT OFFICE ELECTROACOUSTIC TRANSLATOB, INCLUD- ING IMPEDAN CE MATCHING Frank Powell Best, Chelmsford, England, as-

signor to Marcom So Limited, London, Engl Application November 27. 1945, Serial In Great Britain November 27,

undlng Device Company and Claims. (Cl. 177-386) This invention relates to acoustic echo-sounding, and particularly to the coupling of the electro-acoustic translator to the transmitting medium. Though its principal use is in sub-aqueous echo-sounding, and though it will be described in connection therewith, it is not so limited.

It is usual, in marine echo-sounding, to employ one or other of two methods of coupling the electro-acoustic translator to the sea, one of which may be termed the direct and the other the indirect method. By the term electro-acoustic translator is meant the device which, in the use of transmission, translates electrical energy into acoustic wave energy or, in the case of reception, translates received acoustic wave energy into electrical energy. In both methods, the electro-acoustic translator is mounted in a projector, but in the direct method the acoustic translator is in direct contact with the sea water and requires that a hole be made in the shell plating of the ship to which the projector is at-V tached, whilst in the indirect method the pro- `lector is completely enclosed within the structure of the ship and the'electro-acoustic translator is coupled to the sea water outside through the hull of the ship.

This invention relates to the second or indilrect method of coupling, and in applying this method,A it is usual to mount the translator immediately above the ships hull ina tank containing some liquid, generally water, which transmits the energy to the shelL plating of the ship which transmits it to the sea water.

Owing to the fact that the acoustic energy iiected reduced, a much more highly emcient pro- Jector will be made available.

The object of this invention is, therefore, to improve acoustic echo-sounding by making use of a liquid having a much higher acoustic resistance than water and particularly by making use of mercury as an intermediate liquid between the electro-acoustic translator and the ships hull.

The invention is not confined tothe use of mercury but may be extended to any liquid which may have a very noticeably higher acousticreslstance than water.

According to the invention, an acoustic echosounding system comprises an electro-acoustic translator, means deiining an acoustic wave energy transmitting column coupling said translator to the transmitting medium. (for example the sea water) said wave energy transmitting column being a column of liquid of predetermined acoustic resistance which is high relative to the acoustic resistance of water, and a member made of a material having approximately the same acoustic resistance as said liquid interposed as a rigid closure plate member for saidl column between said column and said medium.

Also, according to a feature of the invention, an acoustic lens is included in the liquid column.

A simple formula which gives the ratio of the amount of energy transmitted compared with the amount of incident energy is where R1 and Rz represent the acoustic resistances of the two media concerned. An examination of the formula suggests at once that when the acoustic resistances of two media are of approximately the same values, the transference of energy will he high but where these resistances differ considerably the transference of energy will be low.

The acoustic resistance of Water is 1.43X 105 c. g. s. units while that of steel is 39.3X 105 c. g. s.-

It follows, therefore, that the transference of energy from water to steel will be of a low order, and by calculation is approximately 14%.

If a heavier liquid be introduced between the oscillator and the steel plate of the ship, there will be a considerable improvement in the resultant transfer of energy, and in the case of mercury, the acoustic resistance of which is 193x100, approximately 90% of the energy would be transferred from the mercury column to the steel.

The suitability of mercury can -be more fully appreciated when the transference of energy through steelplates of nite thickness is investigated. Such investigations show, that when the thickness of a plate is of the order of a quarter of a wavelength, or an odd number of quarter wavelengths, the plate becomes largely "opaque" to the particular frequency concerned, whereas when the thickness is approximately equal to a half wavelength, or any number of half wavelengths, the plate is almost transparent to the particular frequency concerned.

Where the thickness of the plate is something less than a quarter of a wavelength, which is generally the case, the amount of energy which is reflected is considerably more than that given in the simple formula above, and investigations will show that the use of mercury as a transmitting agent between the oscillator and the ships hull `has even greater advantages than those previously suggested.

A further point which is of great importance is that owing to the high transference of energy from mercury to various metals, such as steel. nickel, etc., it is possible tol use a metal acoustic lens in a mercury column for concentrating the energy onto a small receiving oscillator situated substantially at the focal point of the lens or of distributing the energy from the small face of a transmitting projector similarly situated over a large area so that a beam may be formed by the interference theorem of transmission from a large area.

This method of producing a beam or of concentrating received energy by means of acoustic lenses is very advantageous where mercury is employed as a substance for the transmission of energy, and various constructions for projectors become possible which otherwise could not be used owing to the relatively low acoustic resistance of any other liquid employed for transmission.

The invention is illustrated in the accompanying drawings.

The translator employed in any embodiment of this invention may be of any suitable kind well known per se and since the invention is not concerned with the construction of the translator itself it is unnecessary to show or describe it in detail. The translator is not shown at all in Figs. 1, 3 and 4 but is merely indicated in conventional form at T in Fig. 2 as va vibratory bar member with an electrical winding thereon.

As illustrated in Fig. 1 the invention may be carried into eiect by welding a mercury-filled casting CA to the steel plating SP of the ships bottom, and, as illustrated in Fig. 2, a steel lens LE may be supported in the mercury column Hg thus constituted.

In an alternative arrangement, illustrated in Fig. 3, a hole HO may be made in the hull of the ship and this hole closed by a metal plate M P one half wave-length or an integral number of half wave lengths in thickness in the material of the plate and at the operating frequency, a mercury filled casting CA being attached to the plating, over the said metal plate. Of course, the plating of the ships bottom may itself be locally thickened to a half wave-length.

A still further method of carrying the invention into effect, illustrated in Fig. 4, similar to that briefly described in the preceding paragraph is by closing the hole with a steel lens LE, or by locally forming the ships plating into a lens.

It is, of course, to be understood, as has been implied elsewhere in this description, that the invention is applicable both for transmission and reception.

What I claim is:

1. An acoustic echo-sounding system comprising an electro-acoustic translator having an acoustic resistance which is high relative to the acoustic resistance of water, means dening an acoustic wave energy transmitting column coupling said translator to the transmitting medium. said wave energy transmitting column being a column of liquid of predetermined acoustic resistance which is high relative to the acoustic resistance oi water, and a member made of a material having approximately the same acoustic resistance as said liquid interposed as a rigid closure plate member for said column between said column and said medium, said transmitting column, said member, and said translator all having substantially the same acoustic resistances.

2. A system as claimed in claim 1 wherein said liquid is mercury.

3. A system as claimed in claim 1 wherein said liquid is mercury and said member is steel.

4. A system as claimed in claim 1 wherein said liquid is mercury and said member is nickel.

5. A ships acoustic echo-sounding system comprising an electro-acoustic translator having an acoustic resistance which is high relative to the acoustic resistance of water, means defining an acoustic wave energy transmitting column coupling said translator to the sea, said wave energy transmitting column being a column of liquid of predetermined acoustic resistance which is high relative to the acoustic resistance of water, and a member made of a material having approximately the same acoustic resistance as said liquid interposed as a rigid closure plate member for said column between said column and the sea, said transmitting column, said member, and said translator all having substantially the same acoustic resistances.

6. A system as claimed in claim 5 wherein said liquid is mercury.

7. A system as claimed in claim 5 wherein said liquid is mercury and said member is steel.

8. A system as claimed in claim 5 wherein said liquid is mercury and said member is nickel.

9. A ships acoustic echo-sounding system comprising an electro-acoustic translator, means defining an acoustic wave energy transmitting column coupling said translator to the sea, said wave energy transmitting column being a column of liquid of predetermined acoustic resistance which is high relative to the acoustic resistance of water, and a member made of a material having approximately the same acoustic resistance as said liquid interposed as arigid closure plate member for said column between said column and the sea, said member being the order of half a wave-length in the material of said plate or closure at the frequency of operation, said transmitting column, said member, and said translator all having substantially the same acoustic resistances.

10. A ships acoustic echo-sounding system comprising an electro-acoustic translator having an acoustic resistance which is high relative to the acoustic resistance of water, means defining an acoustic wave energy transmitting column coupling said translator to the sea, said wave energy transmitting column being a column of liquid of predetermined acoustic resistance which is high relative to the acoustic resistance of water, and a member made of a material having approximately the same acoustic resistance as said liquid interposed as a rigid closure plate member for said column between said column and the sea, said member being constituted by a part of the ybottom-of the ship and there being provided a collimating acoustic lens situated in said liquid column adjacent said member, the electro-acoustic translator being situated at substantially the focal point of said lens, said transmitting column, said member, and said trans- UNTI'ED STATES PATENTS Number Name Date labort al1 havmg substantlally the same acousmc 1,117,766 Berger No 1.7, 1914 res ance' 1,121,986 Davison Dec. 22, 1914 FRANK POWELL BEST. 1,471,547 Chilowsky et al. Oct. 23, 1923 2,384,465 Harrison Sept. l1, 1945 REFERENCES CITED l2,411,541 Hayes NOV.' 26, 1946 5 2,420,676` `Peterson May 20, 1947 The following references are of record in the 2,423,306 Forbes et 1 July 1, 1947 me of this patent: 1 2,427,348 Bond et a1 sept. 16, 1947 Mason Jan. 20, 1948 

